National Historic Chemical Landmarks
Chemists and Chemistry that Transformed Our Lives
®
CCl2F2 + UV light
CCl3F + UV light
Cl + O3
ClO + O
ClO + O2
CClF2 + Cl
CCl2F + Cl
Cl + O2
Chlorofluorocarbons and
Ozone Depletion
University of California, Irvine
American Chemical Society
“Chlorofluoromethanes are being added to the environment in steadily increasing amounts.
These compounds are chemically inert and may remain in the atmosphere for 40-150 years,
and concentrations can be expected to reach 10 to 30 times present levels. Photodissociation
of the chlorofluoromethanes in the stratosphere produces significant amounts of chlorine
atoms, and leads to the destruction of atmospheric ozone.”
— F. Sherwood Rowland and Mario J. Molina, Nature, 1974
“In addition to
their Nobel Prize
winning work
showing that
CFCs break down
the Earth’s ozone
layer, Rowland
and Molina were
key in convincing scientists,
policymakers, and
the general public
about the harmful
effects of CFCs.
Their unprecedented advocacy
ultimately led to
the phasing out
of CFCs worldwide
through the passage of the Montreal Protocol in
1987. The research
of Rowland and
Molina brought
worldwide attention to the impact
of human-contributed pollution on
a planetary scale.
Their work was
among the first
to directly effect
a global shift in
policy, preceding the current
debate on climate
change.”
— Kenneth C.
Janda, professor
of chemistry and
dean, School of
Physical Sciences,
University of
California, Irvine
The language is dry and academic,
as is appropriate for the abstract of
a scientific paper in the prestigious
journal Nature. The research described in the short paper, however,
fell like a scientific bombshell, one
whose repercussions would be felt
around the world. It set off fierce debates, led to a global environmental
treaty restricting the use of a broad
class of chemicals, and changed the
way humans viewed their impact
on Earth’s environment. It also led
to F. Sherwood Rowland (1927-2012)
and Mario J. Molina (*1943) sharing
the 1995 Nobel Prize in Chemistry
with Paul J. Crutzen of the Max Plank
Institute for Chemistry, Mainz, another pioneer in stratospheric ozone
research.
Rowland, a professor of chemistry
at the University of California, Irvine,
and Molina, a postdoctoral fellow
in Rowland’s laboratory, had shown
that chlorofluorocarbons—CFCs—
could destroy ozone, a molecule
made up of three oxygen atoms, O3,
in Earth’s stratosphere. That stratospheric ozone absorbs ultraviolet radiation that otherwise would reach
the surface of Earth. At the time,
CFCs were in wide use in refrigeration, air conditioning and aerosol
spray cans. The compounds are inert
and essentially nontoxic, characteristics that made them well-suited
for these applications. These same
characteristics, however, also made
them a danger to life on Earth.
WIDESPREAD USE OF CFCs
In the 1920s, refrigeration and air
conditioning systems used compounds such as ammonia, chloromethane, propane and sulfur dioxide
as refrigerants. Though effective,
the compounds were toxic and flammable, and exposure to them could
result in serious injury or death.
A team of chemists at Frigidaire
led by Thomas Midgely Jr. (18891944) worked to develop nontoxic,
nonflammable alternatives to the
refrigerants.
The team focused their effort on
compounds containing carbon and
halogens such as fluorine and chlorine. Such compounds were known
to be volatile and chemically inert,
both important properties for the
team studying their use in refrigeration. The first compound they developed was dichlorodifluoromethane,
CCl2F2, which they dubbed “Freon.”
Midgely would receive the Society
of Chemical Industry’s Perkin Medal
for this research in 1937; in 1941, he
was awarded the Priestley Medal,
the American Chemical Society’s
highest award, for his contributions
to chemistry.
By the early 1970s, CFCs were in
widespread use, and worldwide
production of the compounds had
reached nearly one million tons per
year, representing roughly a $500
million slice of the chemical industry.
THE IMPORTANCE OF OZONE
From an environmental standpoint,
ozone is a confusing molecule. In
the troposphere, the region of the
atmosphere from Earth’s surface up
to about 6 miles, ozone is a pollutant
that is a component of photochemical smog. But in the stratosphere,
the region of the atmosphere from 6
to 31 miles, ozone absorbs potentially
damaging ultraviolet (UV) radiation.
As the Royal Swedish Academy of
Sciences put it in its announcement
of the 1995 Nobel Prize in Chemis-
try: “Even though ozone occurs in
such small quantities, it plays an
exceptionally fundamental part in
life on earth. This is because ozone,
together with ordinary molecular
oxygen (O2), is able to absorb the
major part of the sun’s ultraviolet
radiation and therefore prevent this
dangerous radiation from reaching
the surface. Without a protective
ozone layer in the atmosphere,
animals and plants could not exist,
at least not upon land.”
Rowland’s interest in the fate of
CFCs in the atmosphere was sparked
by a talk he heard at a conference in
1972. The speaker discussed results
obtained by James Lovelock (*1919),
a British scientist who had invented
a highly sensitive way to measure
trace gases. Lovelock had measured
trichlorofluoromethane (CFC-11) in
the atmosphere in amounts that
suggested that practically all of the
CFC-11 ever manufactured was still
present in the atmosphere.
Rowland decided to devote a portion
of his research to understanding
the fate of CFCs in the atmosphere.
Although CFCs are inert in the lower
troposphere, Rowland realized that
they can be broken down by UV
radiation once they drift up into the
stratosphere. In late 1973, Rowland
and Molina, who had recently joined
Rowland’s lab, used data from a variety of published sources to calculate
that CFC molecules released near
the surface of Earth would, over decades, wind up in the stratosphere
where UV radiation would split off
chlorine atoms. Each chlorine atom
would react immediately with an
ozone molecule, setting off a chain
reaction that would destroy thousands of ozone molecules. In their
A N AT I O N A L H I S TO R I C C H E M I C A L L A N D M A R K
1979
paper, they estimated that if CFC use
was banned immediately, ozone loss
would go on for years. If CFC production continued, however, ozone loss
would be even greater.
“When we realized there was a
very effective chain reaction, that
changed the CFC investigation from
an interesting scientific problem to
one that had major environmental consequences,” Rowland told
Chemical & Engineering News in an
extensive interview in 2007. “You
don’t often get many chills down
your back when you look at scientific
results,” he added, but that had been
one of those moments.
FROM RESEARCH TO RESISTANCE
In 1976, the National Academies of
Science issued a report affirming
the destructive effects of CFCs on
stratospheric ozone. Congressional
hearings reached similar conclusions, and states and the federal
government began exploring bans
on the use of CFCs in aerosol cans.
The chemical industry maintained
that the data on CFCs and stratospheric ozone were inconclusive and
didn’t warrant drastic action. When
Rowland lectured on CFCs, industry
groups often released statements
disputing his claims. As Molina recalls today, “Sherry [Rowland] was an
established and respected scientist
who regularly gave talks all over the
world. It seemed that, because of his
focus on CFCs and ozone depletion,
he started getting fewer invitations
to speak. That bothered him.”
Rowland and Molina and the other
scientists trying to understand
stratospheric chemistry faced seri-
1989
ous and fundamental challenges.
A significant number of chemical
species were clearly involved in the
interaction of CFCs and ozone in
the stratosphere. Most are highly
reactive and present in only trace
amounts. Their chemistry was difficult to replicate in the laboratory.
Additionally, stratospheric ozone
concentrations fluctuate naturally
by geography and by season. The
stratosphere is not an easy place to
do research in. Measurements of
ozone concentration were carried
out by instruments carried into
the stratosphere by balloons and
aircraft. Ozone was also measured
by instruments on satellites orbiting
Earth, though satellite technology
in the mid-1970s was still rather
primitive.
All of these uncertainties gave critics
of Rowland and Molina’s hypothesis
a lot of material to work with. They
argued, to many very convincingly,
that it simply didn’t make sense to
take action against a class of highly
useful chemicals on the basis of such
flimsy evidence. Industry critics, in
particular, argued that it was one
thing to propose phasing out the
use of CFCs as propellants in aerosol
cans—a relatively trivial use of the
compounds—but quite another to
consider banning their use in refrigerators and air conditioners, where
obvious alternatives to CFCs simply
didn’t exist at that time.
ANTARCTIC OZONE HOLE
The crucial evidence supporting the
CFC hypothesis came from British
scientists working at the Halley
Bay Station of the British Antarc-
2016
tic Survey, who had been taking
ground-based measurements of
total ozone for decades. In 1984,
Joseph C. Farman (1930-2013) and his
colleagues at BAS studied the raw
data and found that stratospheric
ozone had decreased greatly since
the 1960s. In 1985, the scientists
published an article in Nature announcing that stratospheric ozone
over Antarctica was reduced 40%
in September, the end of the austral
winter.
The Antarctic ozone hole, as it came
to be known, made depletion of
the ozone layer a real and present
danger to lawmakers and the public
at large. Predictions of significant
increases in the incidence of skin
cancer resulting from continued use
of CFCs spurred international action.
In 1987, 56 countries agreed under
what became known as the Montreal Protocol to cut CFC production
and use in half. In subsequent years,
the protocol was strengthened to require an eventual worldwide phaseout of the production of CFCs and
other ozone depleting chemicals.
As a result of Rowland and Molina’s
work, humans for the first time
realized that their activities could
affect Earth’s environment on a
planetary scale. As Molina says
today, “It doesn’t matter where CFCs
are emitted. It is a global problem.
What is important is that it led to an
international agreement that solved
the problem.” The saga of CFCs and
the ozone layer holds many lessons
for humanity dealing with the even
larger challenge of global climate
change.
NASA began
measuring Earth’s
stratospheric
ozone layer by
satellite in 1979.
By the time the
Montreal Protocol
went into effect
in 1989, ozone
concentrations
(in Dobson units)
had declined
significantly over
the Antarctic,
enlarging the
ozone hole. Ozone
levels have since
stabilized, but
recovery is still
decades away,
according to
NASA.
Chlorofluorocarbons and Ozone Depletion
A National Historic Chemical Landmark
ACS designated F. Sherwood Rowland and Mario J. Molina’s discovery of CFCs’
harmful effect on ozone as a National Historic Chemical Landmark in a ceremony at the University of California, Irvine, on April 18, 2017. The commemorative plaque reads
At the University of California, Irvine, F. Sherwood Rowland and Mario J.
Molina discovered that chlorofluorocarbons (CFCs) could deplete Earth’s
atmospheric ozone layer, which blocks the sun’s damaging ultraviolet rays.
When the scientists reported their findings in 1974, CFCs were widely used as
refrigerant gases and as propellants in aerosol sprays. Rowland and Molina
convinced skeptical industrialists, policymakers, and the public of the danger
of CFCs. The scientists’ advocacy — and the discovery by other researchers that
the ozone layer over the Antarctic was thinning — led to worldwide phaseout
of CFCs and the development of safer alternatives. For their work, Rowland and
Molina shared the 1995 Nobel Prize in Chemistry with another atmospheric
chemist, Paul J. Crutzen.
About the National Historic Chemical Landmarks Program
ACS established the National Historic Chemical Landmarks program in 1992
to enhance public appreciation for the contributions of the chemical sciences
to modern life in the United States and to encourage a sense of pride in their
practitioners. The program recognizes seminal achievements in the chemical
sciences, records their histories, and provides information and resources about
Landmark achievements. Prospective subjects are nominated by ACS local sections, divisions or committees; reviewed by the ACS National Historic Chemical
Landmarks Subcommittee; and approved by the ACS Board Committee on
Public Affairs and Public Relations.
ACS is a nonprofit organization chartered by the U.S. Congress. With nearly
157,000 members, ACS is the world’s largest scientific society and a global
leader in providing access to chemistry-related research through its multiple
databases, peer-reviewed journals, and scientific conferences. Its main offices
are in Washington, D.C., and Columbus, Ohio.
Acknowledgments:
Written by Rudy Baum.
The author wishes to thank contributors to and reviewers of this booklet, all of
whom helped to improve its contents, especially members of the University of
California, Irvine, Planning Committee; the ACS Committee on Environmental Improvement; and the ACS National Historic Chemical Landmarks Subcommittee.
The ACS Committee on Environmental Improvement sponsored the nomination for this Landmark designation.
Cover photograph: F. Sherwood Rowland (left) and Mario J. Molina in their UC
Irvine lab in 1974. Cover overlay of chemical equations: In the stratosphere, UV
light releases ozone-destroying chlorine atoms from CFCs. Cover photo courtesy UC Irvine. Page three images: 1979 and 1989 images courtesy of NASA’s
Ozone Hole Watch; 2016 image by Jesse Allen, using Suomi NPP OMPS data
provided courtesy of Colin Seftor (SSAI) and Aura OMI data provided courtesy
of the Aura OMI science team. Suomi NPP is the result of a partnership between NASA, NOAA and the Department of Defense.
Designed by Barb Swartz, Design One
Printed by CAS, a division of the American Chemical Society
© 2017 American Chemical Society
American Chemical Society
Allison Campbell, President
Peter Dorhout, President-Elect
Donna Nelson, Immediate Past
President
Pat Confalone, Chair, Board of Directors
University of California, Irvine, Planning
Committee
Tatiana Arizaga, School of Physical Sciences
Donald Blake, Department of Chemistry
Barbara Finlayson-Pitts, Department of Chemistry
Kenneth Janda, Department of
Chemistry and School of Physical
Sciences
Pramod Khargonekar, Vice-Chancellor for Research
Enrique Lavernia, Provost and
Executive Vice-Chancellor
Marijana Lekousis, School of Physical Sciences
Sergey Nizkorodov, Department of Chemistry
James Nowick, Department of Chemistry
Veronique Perraud, Department of Chemistry
Melissa Sweet, Department of Chemistry
Fe Valencia, School of Physical Sciences
Lisa Wingen, Department of Chemistry
ACS Committee on Environmental
Improvement
Anthony Noce, Chair
Laura Pence, Past Chair
Catherine Middlecamp, Program Chair
Melissa Pasquinelli, Secretary
ACS National Historic Chemical
Landmarks Subcommittee
Alan Rocke, NHCL Subcommittee Chair, Case Western Reserve University, emeritus
Mary Ellen Bowden, Chemical Heritage Foundation, retired
Carmen Giunta, Le Moyne College
David Gottfried, Georgia Institute of Technology
Arthur Greenberg, University of New Hampshire
Mark Jones, The Dow Chemical Co.
Diane Krone, Northern Highlands Regional High School, retired
Vera Mainz, University of Illinois at Urbana-Champaign, retired
Seymour Mauskopf, Duke University, emeritus
Andreas Mayr, Stony Brook University
Daniel Menelly, Rochester Museum and Science Center
Michal Meyer, Chemical Heritage
Foundation
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Heinz Roth, Rutgers University
Jeffrey Sturchio, Rabin Martin
Richard Wallace, Armstrong State University
Sophie Rovner, ACS Staff Liaison and NHCL Program Manager
American Chemical Society
National Historic Chemical Landmarks Program
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